1. Field of the Invention
The present invention relates to an oxidation method of oxidizing the surface of a workpiece, such as a semiconductor wafer, and an oxidation system for carrying out the oxidation method.
2. Description of the Related Art
Generally, a semiconductor integrated circuit if fabricated by subjecting a semiconductor wafer, such as a silicon wafer, to processes including a film deposition process, an etching process, an oxidation process, a diffusion process and a modifying process. The oxidation process, for example, oxidizes the surface of a single-crystal or polycrystalline silicon film or a metal film. The oxidation process is used particularly for forming a gate oxide film and an insulating film for a capacitor.
Oxidation processes are classified into atmospheric oxidation processes to be carried out in an atmosphere of a pressure approximately equal to the atmospheric pressure in a processing vessel and low=-pressure oxidation processes to be carried out in a vacuum atmosphere in a processing vessel evacuated at a vacuum in terms of process pressure or into wet oxidation processes using, for example, steam generated by burning hydrogen by an external combustion apparatus, such as a wet oxidation process disclosed in Patent document 1, and dry oxidation processes not using steam and supplying only ozone or oxygen into a processing vessel, such as a dry oxidation process disclosed in Patent document 2.
Generally, insulating films formed by the wet oxidation process are superior to those formed by the dry oxidation process in terms of characteristics including compressive strength, corrosion resistance and reliability. Generally, the atmospheric wet oxidation process is able to form the insulating film at an oxidation rate higher than that at which the low-pressure wet oxidation process. However, the insulating film formed by the atmospheric wet oxidation process is inferior to that formed by the low-pressure wet oxidation process in thickness uniformity.
Conventional design rules for semiconductor integrated circuits are not very severe and hence the foregoing various oxidation processes have been used properly taking into consideration purposes of oxide films, process conditions and equipment cost. Recently, semiconductor integrated circuits need very narrow lines and very thin films, and severe design rules must be applied to designing semiconductor integrated circuits. Consequently, films having higher quality, higher characteristics and higher thickness uniformity have been required in recent years. The conventional oxidation processes are unable to cope with such high requirements.
An oxidation system disclosed, for example, in Patent document 3 carries out a wet oxidation process by supplying hydrogen gas (H2) and oxygen gas (O2) separately into a space at the lower end of a vertical quartz reaction tube, generating steam through the interaction of H2 and O2 in a combustion space formed in a quartz cap, and supplying the steam upward toward wafers to oxidize the wafers. Since H2 is burned in the combustion space, the atmosphere in a downstream part of the space in the processing vessel has a high steam concentration and the atmosphere in an upper part of the space in the processing vessel has a low steam concentration because the steam is consumed as the same flows upward in the processing vessel. Consequently, in some cases, the thickness of an oxide film formed on the wafer is dependent on the position of the wafer in the processing vessel and oxide films of different thicknesses are formed on the wafers, respectively.
A batch-type oxidation system disclosed in Patent document 4 arranges a plurality of semiconductor wafers in a horizontal reaction tube and supplies O2 from one end of the reaction tube into the reaction tube or supplies O2 and H2 simultaneously into the reaction tube to form oxide films on the semiconductor wafers in a low-pressure atmosphere. This oxidation system forms a film in an atmosphere of a comparatively high pressure by a hydrogen-burning oxidation process. Since steam is a principal element of reaction in this oxidation system, it is possible that the difference in steam concentration between an upstream part and a downstream part of the space in the processing vessel with respect to the flowing direction of the gas is excessively large and oxide films having different thicknesses are formed on the semiconductor wafers.
A single-wafer processing oxidation system disclosed in Patent document 5 supplies oxygen gas and hydrogen gas into a processing vessel, generates steam in the vicinity of a semiconductor wafer, such as a silicon wafer, held in the processing vessel through the interaction of the oxygen gas and the hydrogen gas to form an oxide film by oxidizing the surface of the semiconductor wafer. When this single-wafer processing oxidation system is used, oxygen gas and hydrogen gas are supplied into the processing vessel through gas inlets at a short distance between 20 and 30 mm from the semiconductor wafer to generate steam in the vicinity of the surface of the semiconductor wafer through the interaction of the oxygen gas and the hydrogen gas, and a comparatively high process pressure is used. Consequently, the oxide film thus formed on the semiconductor wafer is unsatisfactory in thickness uniformity.
The applicant of the present invention patent application disclosed an oxidation method in Patent document 6. This oxidation method supplies an oxidizing gas, such as oxygen gas, and a reducing gas, such as hydrogen gas, simultaneously into an upstream part and a downstream part, respectively, of a processing chamber, makes the oxidizing gas and the reducing gas interact in a vacuum atmosphere to create an atmosphere containing, as principal elements, active oxygen species and active hydroxyl species, and oxidizes a silicon wafer or the like in this atmosphere.
The oxidation method disclosed in Patent document 6 will be briefly explained with reference to FIG. 4 showing a conventional oxidation system 2. The oxidation system 2 has a cylindrical, vertical processing vessel 6 and a resistance heater 4 surrounding the processing vessel 6. A wafer boat 8 is loaded into and unloaded from the processing vessel 6 through the lower open end of the processing vessel 6 by a wafer boat lifter. The wafer boat 8 holds semiconductor wafers W, such as silicon wafers, in a vertical arrangement. A H2 supply nozzle 10 for supplying H2 and an O2 supply nozzle 12 for supplying O2 are connected to lower parts of the side wall of the processing vessel 6. A discharge port 14 formed in the upper wall of the processing vessel 6 is connected to a vacuum pump or the like.
Hydrogen gas and oxygen gas supplied through the supply nozzles 10 and 12 into a lower part of a process chamber in the processing vessel 6 interact in the processing chamber at a pressure below 133 Pa in the processing vessel 6 to generate active oxygen species and active hydroxyl species. Those active species come into contact with the surfaces of the wafers W as the same rise in the processing vessel 6 to oxidize the surfaces of the wafers W.
Patent document 1: JP-A 3-140453
Patent document 2: JP-A 57-1232
Patent document 3: JP-A 4-18727
Patent document 4: JP-A 57-1232
Patent document 5: U.S. Pat. No. 6,037,273
Patent document 6: JP-B 2002-176052
Oxidation methods disclosed in Patent documents 1 to 6 are capable of forming oxide films of a satisfactory quality in high intrafilm thickness uniformity. However, those oxide films are unsatisfactory in interfilm thickness uniformity. It is inferred that such unsatisfactory interfilm thickness uniformity is due to a high active species concentration in an upstream part of the processing chamber with respect to the flowing direction of the gases and a low active species concentration in a downstream part of the processing chamber. Early solution of problems that may arise due to such unsatisfactory interfilm thickness uniformity is desired in these days when the severity of design rules for semiconductor devices and the reduction of line width film thickness are progressively increasing.
It may be possible to form films in satisfactory interfilm thickness uniformity by heating the wafers held on the wafer boat at different temperatures gradually changing in the direction of arrangement of the wafers by the so-called temperature tilt control. Since the respective temperatures of the wafers held on the wafer boat differ slightly from each other, the different wafers have different heat histories, respectively, and the different heat histories may affect adversely to the characteristics of the films. Accordingly, temperature tilt control is unacceptable.